The host-parasite relationship in experimental syphilis will be studied systematically with a particular focus on the role of immunity. Syphilis represents a major unsatisfactorily controlled endemic infectious disease with epidemic potential. The immune response to the infecting organisms, Treponema pallidum is poorly understood. Our laboratory has been engaged in a basic study of the immune response of the rabbit for over ten years and experimental syphilis for 1 1/2 years. The assays and understanding that has been developed will be applied to experimental T. pallidum infection in the rabbit. The immune system of rabbits with active infection, as well as rabbits resistant to T. pallidum re-infection, will be studied. The assays applied will include serologic tests for syphilis, characterization of mitogenic and specific blastogenic responses to T. pallidum antigens by lymphoid cells and T and B cell distribution, as well as T. pallidum distribution, by fluorescence and immunoelectronomicroscopy techniques. To evaluate the effects of modification of the host on the course of experimental syphilis, neonatal rabbits, cortisone treated rabbits and thymectomized rabbits will be studied. These studies should help define the effect of different immune states upon T. pallidum infection. The antigenic nature of T. pallidum will be analyzed and characterized. These antigens, as well as "slime coat" mucopolysaccharide, will be used to attempt to produce immunity to infection. It is anticipated that these studies will lead to new understanding of syphilis infection and provide insights that eventually can be applied to development of a clinically useful vaccine.